1. Field of the Invention
This invention relates to raindrop sensors for detecting amount of raindrops attached to a plate.
2. Description of the Related Art
Japanese Laid Open Patent Publication No. 59-61752, which has been laid open to public inspection in 1984, discloses a raindrop sensor. As illustrated in FIG. 10, the sensor has a light emitter 52 and a light receiver 53 fixed to a front windshield 51 of a car. Light 57 radiated from the emitter 52 advances while making total reflection in the windshield 51, and gets in the receiver 53. If raindrops 56 are attached to the windshield 51, the light 57 is not totally reflected. Thus, the more the amount of the raindrops 56 is, the less the amount of the light 57 entering the receiver 53 is.
However, this sensor is easily affected by extraneous light, e.g. sunlight, illumination light or the like. Therefore, there is a problem that detection of the raindrops 56 is disabled when the extraneous light is very strong.
Japanese Laid Open Patent Publication No. 59-192651, which has been laid open to public inspection in 1984, discloses another raindrop sensor. As shown in FIG. 11, the sensor has an ultrasonic transmitter 54 and an ultrasonic receiver 55 secured to a front windshield 51 of a car. Ultrasonic waves 50 radiated from the transmitter 54 are reflected at a wall surface of the windshield 5 and go into the receiver 55 after a fixed time. When raindrops 56 are deposited on the windshield 51, the ultrasonic waves 50 become reflected at a boundary surface of the raindrops 56 relative to the air. Thereby, the transmission time of the ultrasonic waves 50 is made longer from the radiation by the transmitter 54 to the detection by the receiver 55.
However, the above sensor can detect only the raindrops 56 just under the transmitter 54 and the receiver 55. As a result, there arises a problem that many pairs of transmitter 54 and the receiver 55 are necessary in case the sensor detects raindrops over a large area such as a front windshield of a car.
Moreover, the transmission time of the ultrasonic waves 50 or time difference is very short from the radiation by the transmitter 54 to the detection by the receiver 55. Subsequently, there is caused another problem that an electric circuit is very complicated and large sized in order to measure the time difference.